Method of making a shaped silicon carbide-silicon matrix composite and articles made thereby

ABSTRACT

A method is described for making a shaped silicon carbide-silicon matrix composite. A confined carbon fiber preform is infiltrated with sufficient molten silicon metal at a temperature in the range of from about 1400° C. to about 1800° C. in an inert atmosphere or vacuum. Silicon carbide powder can also be incorporated in the preform structure.

This is a division of application Ser. No. 100,579, filed Dec. 5, 1979,now U.S. Pat. No. 4,240,835, which is a continuation-in-part of ourcopending application Ser. No. 946,718, filed Sept. 28, 1978, nowabandoned, which is a division of our application Ser. No. 572,969,filed Apr. 30, 1975, now U.S. Pat. No. 4,141,948, as acontinuation-in-part of application Ser. No. 354,106, filed Apr. 24,1973 now abandoned, and all assigned to the same assignee as the presentinvention.

This invention relates to a method of making shaped siliconcarbide-silicon matrix composite articles and to the articles madethereby.

Prior to the present invention, the fabrication of parts for hightemperature applications, such as turbine blades and vanes for gasturbines, etc., presented a formidable challenge to the heat engineindustry. As higher operating temperature requirements were identifiedsuch as in excess of 1200° C., increasing attention was directed toceramics, such as silicon carbide. However, the design problemsassociated with the brittle nature of these materials and difficulty offabrication presented severe obstacles.

Those skilled in the art know that structural ceramics are eitherhot-pressed and machined to final shape or sintered to final shape. Theformer process is time consuming and expensive; the latter is subject todistortion and dimensional uncertainties. Other methods of fabricationinclude reaction-binding as taught by Forrest U.S. Pat. No. 3,495,939.Finely divided silicon carbide and carbon are blended with a binder andextruded to a particular shape, such as a tube. The shaped structure isheated in air to produce a porous shaped structure. The structure isthen contacted in a vertical position with molten silicon metal or vaporresulting in silicon infiltration. As a result, a shaped silicon carbidebody is produced having valuable characteristics. However, the procedureof shaping the porous body by extrusion is limited, particularly whereseveral parts are required and the shape is somewhat complex.

A further refinement in attempting to prepare a silicon carbide ceramicis shown by Wakefield U.S. Pat. No. 3,459,842. Wakefield utilizes amixture of powdered silicon and silicon carbide whiskers and places themixture into a quartz vessel. As taught by Wakefield, a reinforcedsilicon composite material is produced which can be made with alignedsilicon carbide crystals by elevating the temperature slightly above themelting point of silicon and drawing the quartz vessel. The procedure ofWakefield, however, is therefore limited to the use of costly siliconcarbide whiskers. In addition, alignment of the silicon carbide crystalsas taught by Wakefield can only be achieved by using a vessel made outof a drawable material such as quartz.

An object of the present invention, therefore, is to produce a shapedsilicon carbide refractory article within a close tolerance of thedesired shape such as 1/2% by a simple efficient procedure.

Another object of the present invention is to provide a method forproducing a variety of shaped silicon carbide objects having a broadspectrum in properties ranging from a material having flowcharacteristics and ductility similar to silicon, to a material as hardas pure silicon carbide.

A further object of the present invention is to produce a shaped pluralphase silicon carbide ceramic exhibiting fibrous composite-likeproperties because of the aligned nature of the silicon carbide phase.

A still further object of the present invention is to provide a shapedsilicon carbide refractory article exhibiting improved physicalproperties.

These and further objects, features and advantages of the inventionbecome apparent from the following description and drawing in which:

The drawing shows a front view of a mold containing a shaped carbonfiber structure and the mold is in a supporting structure. Above themold there is shown a charge of powdered silicon.

More particularly, there is shown in the drawing at 10, a supportingstructure which can be used and is made from graphite such as ArmcoSpeer 580, which can be readily machined to a particular shape. There isshown at 11 a mold, which also can be made from Armco Speer 580, orother suitable material capable of withstanding elevated temperaturesand resistance to molten silicon. At 12 there is shown a mold cavityfilled with a preformed carbon fiber structure such as a preform; and at13 and 14 there are shown carbon fiber wicks. Vent holes 16 and 17 allowfor the escape of hot gases from the mold which can exit out of vent 19.A connector, or mold forming means at 15 having a threaded end can beused to confine the molten silicon formed by heating the powderedsilicon charge at 18 in the mold cavity.

According to certain embodiments of the present invention, there isprovided a method for making a shaped ceramic part capable of providinga 0.1"×0.1" section having an average 3 point bend test tensile value offrom 30 KSI to 99 KSI when tested over a 5/8" span at a temperature of25° C., where said shaped ceramic has from about 4% to 30% by weightcarbon in the chemically combined form, or as a mixture of chemicallycombined carbon and elemental carbon, which comprises uniformlyinfiltrating with molten silicon at a temperature of from about 1400° C.to about 1800° C. under an inert atmosphere, or in a vacuum, a shapedcarbon fiber structure having an average specific gravity of from about1.3 to 2 which is confined in and substantially fills a mold cavityshaped substantially to the shape of the ceramic part until theinfiltrated silicon substantially fills the mold cavity and thereafterseparating the resulting ceramic part from the mold.

As used hereinafter, the term carbon fiber or filaments includescommercially available carbon fiber as previously defined. The carbonfiber includes, for example, "high strength" graphite having a tensilepsi of typically 10⁵ psi, a modulus of 20×10⁶ psi and a carbonizeddensity of 1.6 g/cc as shown by Johnson et al Pat. No. 3,412,062.Preferably the carbon fiber has a specific gravity of about 1.3 to 1.5as calculated from dimensional measurements and weight and includes, forexample, WYK braid, WYB tow of Union Carbide Corp. and other carbonizedfibers derived from rayon or regenerated cellulose fibers such as carbonfelt and still other carbonizable fibrous materials. In addition tocarbonized rayon fibers any carbon fibers having a specific gravity asdefined above derived from other polymeric materials such aspolyacrylonitrile, polyacetylene, such as shown by Krutchen U.S. Pat.No. 3,852,235 assigned to the same assignee as the present invention,polyvinyl chloride, polyvinyl acetate, etc., can be employed. The term"preform," as used hereinafter, is preferably a shaped structure oforiented carbon fibers such as a prepreg which can further includecarboneous residues of other carbonized materials. To form a preform, acarbon fiber tow, braid, flock, felt, mat, or cloth is treated withmolten wax or other binder such as cellulose nitrate, polyesters, epoxyand other resinous binders, colloidal graphite, etc.

It has also been found that significant amounts of conventional siliconcarbide powder optionally can be incorporated in the final shapedsilicon carbide refractory article without any serious deterioration ofthe desired improved physical properties in said final article. Inaccordance with the present invention, the silicon carbide powder cansimply be mixed with the carbon fiber and a binder to form the preformand with said additive serving to substitute for part of the moltensilicon thereafter infiltrated into said preform. The infiltrated moltensilicon chemically reacts with the carbon fiber to produce alignedsilicon carbide crystals in a silicon metal matrix and with the siliconcarbide crystals already present in the preform being dispersed in thesilicon matrix. Examination of the microstructure in this type finalceramic article has revealed that the silicon carbide crystalsoriginally present in the preform have retained the conventionalhexagonal or α crystalline structure whereas the aligned silicon carbidecrystals formed by infiltration of the preform with molten siliconexhibit a cubic or β crystalline structure. Such incorporation ofsilicon carbide powder in the final ceramic article is not onlybeneficial from a cost standpoint but also enhances a more uniformmicrostructure and imparts greater dimensional stability during themolding operation. Additionally, the incorporation of silicon carbideprovides a moderating effect upon the temperature increase resultingfrom the exothermic reaction between silicon and carbon that occursduring the initial infiltration and conversion operations. In thespecific embodiments hereinafter illustrated, up to 25 weight percentsilicon carbide powder based on the weight of the final ceramic articlehas been found not to degrade the desired final physical properties toany great extent, and still greater proportions may be found usefuldepending upon the final microstructure and physical properties beingsought.

In the broadest sense, therefore, the improved ceramic article of thepresent invention comprises aligned silicon carbide crystals in asilicon metal matrix, which are aligned in the same manner as were thecarbon fibers in the preform structure, and which can further containsilicon carbide crystals being dispersed in the silicon metal matrix.Since the preform structure can either employ a general parallelrelationship between the carbon fibers as is obtained with a tow, braidor cloth as well as a general non-parallel carbon fiber relationshipwhich is exemplified with a flock, felt or mat fiber arrangement, thealignment of silicon carbide crystals in the final ceramic articlevaries accordingly. The parallel fiber and crystal arrangement is to bepreferred if mechanical strength requirements for the improved ceramicarticle are critical. As previously noted, the present improved ceramicarticle can also be further characterized in having from about 4% to 30%by weight carbon in the chemically combined form, or as a mixture ofchemically combined carbon and elemental carbon. The improved physicalproperties of the present final ceramic article are represented by anelastic modulus of from about 30×10⁶ psi to about 60×10⁶ psi and whichis further accompanied by a permanent elastic strain when measured at atemperature of at least 1000° C. from 0% to about 6% in value. Thedensity of said improved ceramic article lies in the range of about 2.3to 3.0 grams/cc as measured by the conventional weight and volumedisplacement measurement technique.

In one embodiment of the invention, a mold is precision machined toprovide a mold cavity shaped to accommodate such parts as gas turbineshroud sections or flame holders, aircraft engine shroud sections, gasturbine transition pieces, diesel engine parts such as pistons andrings, or preignition cups or cylinder liners, heat exchange pipes, hotprocessing dies, combustion liners, fusion reactor hardware, wearresistant tiles, etc. Still other automotive parts can be produced inthe same manner to include engine turbochargers and manifolds as well asbrake rotors. Unrelated product applications for the improved ceramicarticle of the present invention include containers for the chemicalindustry to produce chemicals such as inorganic phosphors at elevatedtemperatures and/or corrosive environments as well as cookware and ovenparts for the home. If desired, the mold can be treated, for example,sprayed, with a boron nitride release agent. The use of boron nitride isshown in the application of William B. Hillig, Ser. No. 419,286, filedNov. 27, 1973, now abandoned and assigned to the same assignee as thepresent invention.

There is placed in the mold, a carbon fiber structure such as a preformmachined or fashioned substantially to the shape and size of the moldcavity. The total weight of carbon fiber per unit of mold cavity volumecan vary widely depending upon the nature of the carbon fiber and mannerby which the carbon fiber is aligned in the preform. In certaininstances a positive pressure can be imposed on the carbon fiberstructure in the mold cavity to conform it to the shape of the mold bythe use of a mold screw or an external means for sealing the mold.

As a result of the use of carbon fiber wicks, as shown in the drawing,infiltration of molten silicon into the mold can be facilitated. Holediameters as small as 10 mils to 125 mils can be advantageously usedwith wicks to avoid the subsequent formation of excessively large nibs.Those skilled in the art know that small nibs can be removed by a simplefinishing operation. In the absence of wicks, hole diameters of at least3/8" can be used which result in nibs large enough to require a separatemachining step. In addition, silicon rich areas can be formed on thesurface of the part where the large nib was attached resulting insurface variations.

The mold can be placed in the support structure as shown in the drawing.A charge of powdered silicon can be placed above the mold and the wholeplaced in an oven. The oven can be evacuated to pressures of 3 to 5×10⁻²torr and optionally with an inert gas such as argon or nitrogen etc., torender the oven atmosphere substantially non-oxidizing.

The charge can be heated to a temperature of 1400° C. to 1800° C.Infiltration of the shaped carbon structure can be accomplished over a 1to 60 minute period or more and preferably 5 to 20 minutes. Afterallowing the mold to cool a temperature of about 20° C., the part can bereadily separated. In the event a release agent is not used on the moldsurface, the mold can be broken to separate the shaped ceramic.

As shown in Report No. 74CRD282 of the Technical Information Series ofthe General Electric Company, Corporate Research and Development,November 1974, scanning electron microscopy has established that theshaped ceramic of the present invention can be composites of alignedsilicon carbide crystals in a silicon metal matrix, which are aligned inthe same manner as were the carbon fibers in the preform structure. Aparallel alignment of said carbon fibers provides improved tensileproperties as was previously indicated. It has been found that siliconpenetration of the carbon fibers occurs most readily along the directionof the fibers and less readily transverse to the fibers. Depending uponthe volume fraction of carbon fibers used in the prepreg or preform, acorresponding volume fraction of aligned silicon carbide crystalssurrounded by domains of silicon will be generated in the resultingsilicon carbide-silicon matrix composite.

There can be present from about 5% to about 75% volume percent ofaligned silicon carbide crystals based on total composite volume of thesilicon carbide-silicon matrix composites of the present invention. At atemperature of at least 1000° C., the composites of the presentinvention also can exhibit from 0% to 6% permanent plastic straindepending upon whether there is a high volume fraction of alignedsilicon carbide crystals, or a low volume. Plastic strain of up to 6%can allow for the relief of localized stresses of sufficient strengthcan lead to failure. The improved silicon carbide-silicon composite ofthe present invention exhibits an elastic modulus of from about 30×10⁶psi to about 60×10⁶ psi.

In order that those skilled in the art will be better able to practicethe invention, the following examples are given by way of illustrationand not by way of limitation. All parts are by weight.

EXAMPLE 1

A carbon fiber preform was prepared from low modulus WCA carbon cloth ofUnion Carbide Corporation using an aqueous colloidal suspension ofgraphite as a binder. The density of the fiber was approximately1.38-1.48 grams/cc and the total weight of fiber in the preform after ithad been machined to a 2.5" diameter disk as shown in the drawing wasabout 11 grams.

A 3" diameter mold was machined out of Speer 580 graphite having a moldcavity of about 2.5 inches and a 0.42 inch thickness. Four 0.125diameter infiltration holes were drilled into the top half of the moldand 0.125 inch diameter vent holes were drilled into the bottom half ofthe mold. Carbon fiber wicks in the form of WYK braid were inserted intothe infiltration holes and protruded about 0.125 inches from the top ofthe mold. The inside surface of the mold was treated with a boronnitride powder in a form of an aerosol spray.

The carbon fiber prepreg was then placed in the mold, and the mold wasthen placed in a supporting structure as shown in the drawing made fromArmco Speer 580 graphite which had been precision machined to thespecifications of the mold. A charge of powdered silicon was then pouredon top of the mold surface. In estimating the amount of silicon, therewas employed up to about a 15% excess of that amount of silicon requiredto fill the mold cavity in the molten state.

The mold and supporting structure was then placed in a furnace which wasmaintained under a vacuum of about 1×10⁻² torr. A pressure of from1×10⁻² torr to 3 torr also was operable. The furnace was maintained at atemperature of about 1600° C. It was found that the silicon powderconverted to molten silicon in about 15 minutes and it was allowed toinfiltrate the carbon fiber prepreg. After an initial cooling period,the mold and supporting structure was removed from the furnace andallowed to cool under atmospheric conditions. The mold was then openedand there was obtained a disk which conformed within 0.2% of thedimensions of the mold cavity. Based on method of preparation, the diskwas a silicon carbide, silicon ceramic having about 16% by weight carbonin the chemically combined form, or as a mixture of chemically combinedcarbon and elemental carbon and about 84% by weight of silicon.

A 0.1"×0.1"×1.0" section of the above composite was removed with adiamond cutting wheel and subjected to a 3 point bend test, which isdescribed as follows:

The specimen is placed on steel rollers 5/8" spaced apart in a testingmachine and loaded through a steel roller at a rate of 0.005 in/min. Thefracture load is obtained from the autographic record of test, as shownin ASTM E4-72, Verification of Testing Machines. The stress iscalculated from the following elementary stress formula as follows:##EQU1## where P is the fracture load, 1 the span (5/8"), b the specimenwidth (0.1") and h the thickness (0.1").

The 3 point bend test value of the above specimen was found to be about38 KSI. Another specimen of the above silicon carbide-silicon ceramicwas also examined with a scanning electron microscope after about 40microns of silicon was etched away in a hydrogen fluoride-nitric acidetching solution. It was found that the ceramic resembled a compositewith silicon carbide crystals substantially aligned in a patterncorresponding to the carbon fibers where the silicon carbide crystalswere surrounded by domains of silicon metal to produce a siliconcarbide-silicon matrix composite. The volume fraction of the patternedsilicon carbide crystals was estimated to be 75% based on the totalcomposite. The silicon carbide-silicon matrix composite also had anelastic modulus of about 48×10⁶ psi. It also had a density of about 2.8grams/cc.

Those skilled in the art would know that, based on the above procedurefor making precision molding high performance shaped ceramics, that theimproved silicon carbide-silicon matrix composites of the presentinvention would be suitable if made in the form of a gas turbine shroudor aircraft engine shroud sections.

EXAMPLE 2

The procedure of Example 1 was repeated except that a mold was machinedhaving a 6"×6" square×1/4" thick cavity which was slightly curved to a48" radius. The mold was charged with WDF Union Carbide graphite feltamounting to about 11 grams of carbon. There was obtained a siliconcarbide-silicon matrix composite having about 4% by weight of carbon inthe chemically combined form or as a mixture of chemically combinedcarbon and elemental carbon and about 96% by weight of silicon. Thecomposite had a density of about 2.4 grams/cc.

A specimen of the composite was removed as in Example 1 and it showed a3 point bend test value of about 25-30 KSI. It had a modulus of about35×10⁶ psi and contained patterned silicon carbide crystals having avolume fraction of about 9% by volume. It also exhibited a plasticstrain of about 1 to 6% when measured at a temperature between 1000° C.to 1350° C.

EXAMPLE 3

In accordance with the procedure of Example 1, a mold having a cavity ofabout 150"×178"×3" long was made and charged with about 1.92 grams ofUnion Carbide WYD carbon fiber tow having a density of about 1.38grams/cc. Based on method of preparation there was obtained a shapedceramic structure having about 26% by weight of carbon in the chemicallycombined carbon and elemental carbon and about 74% by weight of silicon.The ceramic also had a density of about 2.92 grams/cc.

A 3 point bend test showed that the ceramic had a tensile strength ofabout 70 KSI. The modulus of the ceramic was about 57.5×10⁶ psi.Examination under a scanning electron microscope of an etched specimenas described in Example 1 showed that the composite had a volumefraction of about 72% of silicon carbide crystals which weresubstantially aligned in the same direction as the carbon fibersoriginally employed in the form of a carbon fiber tow. Those skilled inthe art would know that a silicon carbide-silicon matrix compositehaving the aforementioned characteristics would be ideally suited inhigh temperature applications such as parts for aircraft shroudsections.

EXAMPLE 4

The procedure of Example 1 was repeated except that a variety of carbonfiber was used having a specific gravity over a range of from 1.3 to 1.5and a higher range of specific gravity of between about 1.6 to 2. Thepurpose of the investigation was to determine whether the nature of thecarbon fiber with respect to its specific gravity related to theultimate tensile properties in the final silicon carbide-silicon matrixcomposite with respect to KSI values as determined by theabove-described 3 point bend test. The weight percent of the carbonemployed in making the composite was about 25% in each instance. Thedensity of the WYK braid representing so-called "low density" fibers hada density between 1.3 to 1.5 grams/cc, while the Morganite I andMorganite II representing the "high" density fibers had a densitybetween 1.6 to 2 grams/cc. The following results were obtained whereroom temperature shows the KSI values under atmospheric conditions and1000° C. and 1200° C. show the results obtained at elevatedtemperatures; in addition silicon is also shown to illustrate theresults achieved with the improved silicon carbide-silicon matrixcomposites of the present invention.

    ______________________________________                                        KSI VALUES                                                                             25° C.                                                                           1000° C.                                                                        1200° C.                                   ______________________________________                                        Low density                                                                              43          41       42                                            High density                                                                             22          34       --                                            Silicon    13           7       14                                            ______________________________________                                    

The above results establish that optimum results are achieved inaccordance with the practice of the invention when low density carbonfibers are used.

EXAMPLE 5

The procedure of Example 1 was modified to produce the ceramic articlewith physical dimensions of 1.5 inches by 6.5 inches by 0.125 inch in asuitable size mold cavity. A carbon fiber prepreg was prepared from aphysical mixture containing 75 weight percent crushed carbon felt and 25weight percent silicon carbide powder of 80 mesh size to which had beenadded an epoxy resin binder. This prepreg was placed in the mold cavityand infiltrated with molten silicon at 1550° C. for approximately 45minutes in the same manner described in said Example 1. From said finalceramic plate, specimens were removed for physical measurement ofdensity and elastic modulus which are reported below:

    ______________________________________                                        Sample   Density(Grams/cc)                                                                           Youngs Modulus(PSI)                                    ______________________________________                                        1        2.87          45.5 × 10.sup.6                                  2        2.90          48.8 × 10.sup.6                                  3        2.85          44.7 × 10.sup.6                                  ______________________________________                                    

As can be noted from the above reported elastic modulus values, asuperior modulus is exhibited as compared with Example 2 which did notincorporate silicon carbide powder with carbon felt to prepare theprepreg.

EXAMPLE 6

The procedure of Example 5 was repeated except that the carbon fiberprepreg was produced from a physical mixture containing only 10% byweight silicon carbide powder in said mixture. The density and elasticmodulus values measured upon the final ceramic plate are as follows:

    ______________________________________                                        Sample   Density(Grams/cc)                                                                           Youngs Modulus(PSI)                                    ______________________________________                                        1        2.99          56.8 × 10.sup.6                                  2        2.98          55.8 × 10.sup.6                                  3        3.03          55.9 × 10.sup.6                                  ______________________________________                                    

It can be noted from the above reported density and elastic modulusvalues as compared with the preceding example that both physicalproperties are improved at a lower content of the silicon carbide powderadditive.

Although the above examples illustrate only a few of the very manyvariables which can be employed in the practice of the invention as wellas the types of improved silicon carbide-silicon matrix composites, itwould be understood that the true scope of the invention and compositesmade thereby can be more fully appreciated when the above examples areread along with the description preceding these examples.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. A method for making a silicon carbide-siliconmatrix ceramic of the desired shape which comprises:(a) infiltratingmolten silicon at a temperature of from 1400° C. to 1800° C. into ashaped carbon fiber part which further includes silicon carbide crystalsdispersed therein while said part is substantially contained in a moldin a non-oxidizing atmosphere and under reduced pressure, and (b)separating the resulting silicon carbide ceramic from the mold where thefiber used in (a) has a specific gravity of from about 1.3 to 1.5.
 2. Amethod as in claim 1 where the infiltration of the molten silicon isaccomplished under a pressure of from 1×10⁻² torr to 3 torr.
 3. A methodas in claim 1, where the carbon fiber is in the form of elongated carbonfilaments in general parallel relationship.
 4. A method as in claim 1,where the carbon fiber is in the form of carbon fiber cloth.
 5. A methodas in claim 1, where the carbon fiber is in the form of carbon fibertow.
 6. A method as in claim 1, where the mold is treated with boronnitride prior to the infiltration of the molten silicon.
 7. A method asin claim 1 where carbon fiber wicks are used in the mold to facilitatethe flow of molten silicon therein.
 8. A method as in claim 1 where saidshaped ceramic part has from about 4% to about 30% by weight of carbonin a chemically combined form or as a mixture of chemically combinedcarbon and elemental carbon.